1. Field of the Invention
This invention relates to an apparatus for detecting moisture on a substrate, and in particular, to a moisture sensor mounted on a windshield that senses moisture and actuates a windshield wiper motor.
2a. Technical Considerations
When operating a vehicle, it is important to maintain a clear undistorted viewing area through the windshield of the vehicle. Windshield wiper mechanisms are provided to clear selected portions of the windshield of water and/or dirt that may obscure the vehicle operator's vision.
Moisture sensors have been used to sense rain on windshields. These sensors, which include electrically conductive members having a Protective coating on the exterior surface of the windshield, automatically actuate a windshield wiper motor to remove the water and clear the vision area. The conductive members are generally arranged in a fixed relationship to form variable capacitors whose output varies as a function of moisture absorbed by the overlaying protective coating.
2b. Patents of Interest
U.S. Pat. No. 4,164,868 to Suntola teaches a capacitive humidity transducer having an electrically non-conductive base carrying at least a pair of spaced, electrically conductive coatings along the major surface of the base. A dielectric film which is active with respect to water absorption is also carried by the base and covers at least a portion of the coatings. The dielectric film has a dielectric constant which varies as a function of the extent to which water has been absorbed by the film. An outer, electrically conductive, water-permeable layer is supported by the dielectric film. The dielectric film maintains the outer layer permanently out of contact with at least one of the coatings so that it is possible to measure a capacitance between these coatings, which is indicative of humidity in the atmosphere.
U.S. Pat. No. 4,429,343 to Freud teaches a humidity-sensing element having two sets of interdigitated, thin film platinum fingers deposited on the surface of a glass substrate. The film is covered by a coating of water-absorbing material such as cellulose acetate butyrate or silicone rubber. The humidity sensitivity of the sensor results from the humidity-related dielectric constant change which occurs in the coating over the fingers. As this dielectric constant changes, so does the capacitance between the interdigitated fingers.
U.S. Pat. No. 4,639,831 to Iyoda teaches a transparent sensor for detecting rain on window glass located within the wiping area on the exterior surface of the window glass. The sensor includes a pair of spaced apart electrodes having interdigitated finger members that are insulated electrically from each other by a transparent insulating protective film. The interdigitated members form capacitors having variable capacitance. When a drop of water accumulates on a portion of the protective coating between a pair of finger members, the capacitance of the capacitor becomes greater than the normal capacitance because the dielectric constant of the drop of water on the protective coating is greater than the dielectric constant of air. Accordingly, as the number of drops of water on the protective coating increases, the total capacitor output increases.